Deliberate dissociation of ions via collision-induced dissociation (CID) is a useful technique for verification of molecular entities in mass spectrometers including those based on ion trapping. Collisional activation (CA), which constitutes the first step of CID (dissociation being the second step), can be effected in ion trapping instruments by resonantly exciting ions at one or more of their characteristic frequencies of motion within the trapping field.
Because the frequencies of ion motion are mass-to-charge dependent, the use of a single excitation frequency makes collisional activation in ion trapping instruments mass-to-charge specific. Consequently, a priori knowledge of the mass-to-charge ratio for a selected ion has been required in order that the excitation frequency can be correctly adjusted. Furthermore, the frequencies of ion motion are affected by the presence of other ions of like charge (i.e., space charge) which limits the usefulness of the single frequency excitation methods commonly used in conventional ion trapping instruments.
Single frequency excitation methods as used in ion traps employ a discrete, precisely-tuned frequency to excite a limited number of ions possessing a particular mass-to-charge ratio. Also, in many conventional ion trapping applications, the number of ions is difficult to control which necessitates frequency adjustments that are generally tedious and time-consuming. Furthermore, the extent of fragmentation, which provides structural information, is often limited because once the parent ion becomes fragmented, the first generation product ions fall out of resonance and typically do not undergo further fragmentation. Further background information can be found in the following references, all of which are incorporated herein by reference: J. E. Fulford, et al., "Radio-frequency mass-selective excitation and resonant ejection of ions in a three-dimensional quadrupole ion trap", Journal of Vacuum Science and Technology, 17(4), 829 (1980); R. B. Cody, et al., "Collision-Induced Dissociation with Fourier Transform Mass Spectrometry", Analytical Chemistry, 54, 96(1982); J. N. Louris, et al., "Instrumentation, Applications and Energy Deposition in Quadrupole Ion Trap Tandem Mass Spectrometry", Analytical Chemistry, 59, 1677 (1987); and Syka, et al. "Method of Operating Ion Trap Detector in MS/MS Mode", U.S. Pat. No. 4,736,101.
Yates et al. have described the use of rapid frequency pre-scans over a narrow mass range (1.7 amu) to determine empirically the parent ion resonant frequency in an automated fashion. These workers have also described the use of a supplementary signal with a range in frequency of 10 kHz, equivalent to a mass/charge range of 1.7 at a q.sub.z value of 0.3. Both approaches were found to deal effectively with the space charge problem but neither deals with the problems of limited fragmentation and inability to fragment ions simultaneously over a range of mass/charge values. Further information can be found in N. A. Yates et al., "Resonant Excitation for GC/MS/MS in the Quadrupole Ion Trap via Frequency Assignment Pre-Scans and Broadband Excitation," Proceedings of the 39th ASMS Conference on Mass Spectrometry and Applied Topics, May 1991, Nashville, Tenn., p. 132.
Random-noise excitation has been used in Fourier transform ion cyclotron resonance mass spectrometry as part of the final detection/mass analysis step, but not as a means for inducing fragmentation of ions. Random noise was used to kinetically excite all ions in the ion cyclotron resonance mass spectrometer to higher orbits in order to allow their detection by the image currents they induce in receiver plates. In such an application, vacuum conditions are necessary; collisions with background or bath gases have a pernicious effect upon the detection method. Further information can be found in C. F. Ijames, et al., "Fourier Transform Mass Spectrometry Using Random-Noise Excitation", Chemical Physics Letters, 108, 58 (1984).
Non-selective collisional activation was used in the rapid screening technique described in S. A. McLuckey, et al., "Simultaneous Monitoring for Parent Ions of Targeted Product Ions: A Method for Rapid Screening Using Mass Spectrometry/Mass Spectrometry", Analytical Chemistry, 62, 56 (1990), which is incorporated herein by reference. The experiments reported therein were carried out with a non-trapping instrument and illustrate the usefulness of non-selective, universal collisional activation.
There is a need for apparatus and a method for collisional activation ion trapping which overcomes the problem of frequency shifts due to space charge and enables simultaneous kinetic excitation of trapped ions having a wide range of mass-to-charge ratios. There is also a need for apparatus and a method for collisional activation ion trap mass spectrometry wherein CID can be performed in a universal manner without time-consuming frequency adjustments.